1. Field of the Invention
This invention relates to a wireless communication network in a cellular system formed by a plurality of base stations, which broadcasts information using downlink common channels, transmits information using downlink dedicated channels and receives information using uplink dedicated channels. More particularly, the invention relates to a wireless communication network for broadcasting high priority information using downlink common channels.
2. Description of the Related Art
A prior technique relevant to transmission of high priority information in the wireless communication network has been disclosed by JP-A-10-41875, which describes a communication system and a portable telephone terminal having an function of automatic emergency communication. This terminal changes its transmission method when used for emergency communication so as to transmit high priority information using uplink channels.
Another prior technique has been disclosed by JP-A-10-13336, which describes a public business digital mobile communication system. This system can broadcast high priority information without interrupting individual communications of mobile stations by using idle communication channels or compulsory communication channels in order to transmit and broadcast emergency information, or high priority information.
Of the above-described two techniques, the former is directed to change the transmission method via uplinks, whereas the latter is directed to broadcast by using idle communication channels or compulsory communication channels.
As shown in FIG. 22, each of the base stations of known wireless communication networks having a broadcasting function comprises an uplink dedicated channel reception unit 1, a downlink dedicated channel reception unit 2 and a downlink common channel transmission unit 3.
Normally, a wireless communication system is designed in such a way that the cell radius of an uplink and that of a downlink are substantially equal to each other and base stations are provided for the system in order to realize bidirectional communication via the uplink and the downlink.
Particularly, in W-CDMA (wide band-code division multiple access) type wireless communication systems, the cell radius, or the coverage area, and the wireless communication system capacity qualitatively show a relationship of tradeoff. For this reason, the wireless communication system is designed and regulated so as to make the wireless communication system capacity as large as possible for both the uplinks and the downlinks in order to realize the coverage area that is presumably necessary and sufficient.
Additionally, wireless communication systems have been made to operate as communication systems and, at the same time, as broadcasting systems as in the case of MBMS (multimedia broadcast & multicast systems) in recent years. The functional feature of broadcasting system is realized by using downlinks as common channels for broadcasting information. No problem arises to wireless communication systems that are designed and regulated to operate as broadcasting systems for broadcasting information to the coverage so long as the information to be broadcast has an ordinary level of priority.
However, as the first problem of the prior art, it is not possible to convey the broadcast information quickly and reliably to mobile terminals out of the designed coverage of such a wireless communication system if there occurs a situation where high priority information needs to be broadcast because the system is primarily designed as wireless communication system. Particularly, a place deep in a large building that gives rise to a large propagation loss can often be out of the coverage of the uplink because of the upper limit of the transmission power of the mobile terminal and hence it is also out of the coverage of the downlink. Then, there arises a problem that no broadcast information is conveyed to mobile terminals at places located deep in buildings. Additionally, many scarcely populated areas are designed to be out of the coverage of wireless communication systems because of the profitability of the wireless communication system in addition to a large propagation loss of such areas. Thus, there also arises a problem that no broadcast information is conveyed to mobile terminals of scarcely populated areas.
Conceivable circumstances where high priority broadcast information is transmitted include those where the meteorological agency requests wireless communication systems to broadcast an emergency alarm for exercising vigilance over an earthquake and a tsunami and urging evacuation of a specific area. Other such circumstances may be those where one or more than one responsible administrative ministries and/or agencies request wireless communication systems to broadcast an emergency alarm for exercising vigilance over a terror, a missile attack, an attack of biological weapons or toxic gas, a risk of widespread infection of a highly infectious disease, an accident of a nuclear power plant or the like and urging evacuation. However, high priority broadcast information is not limited to such emergency alarms. High priority broadcast information may be advertisement for a discount sale in a limited time zone that is requested to broadcast by a retailer.
High priority broadcast information may be requested to be or have to be conveyed reliably to mobile terminals at places deep in buildings and scarcely populated areas. However, no technique has been available to date to wireless communication systems for reliably broadcasting such high priority information.
As the second problem of the prior art, it is not possible to estimate the amount of the monetary loss that arises to a wireless communication system when it is forced to broadcast high priority information. Then, it is not possible for the wireless communication system to provide a reliable basis for the charge to be made to the party that requested to broadcast high priority information.